About Eyepieces

An eyepiece is essentially a magnifier. It magnifies the image that the main mirror or lens produces.

Considering the fact that an eyepiece is usually a much simpler optical product than, for example, a pair of binoculars, it is surprising how expensive some of them are.

You can get an excellent telescope for the cost of some of these eyepieces. Before you consider spending $600 or so for an eyepiece, consider upgrading your telescope. You will see more with a bigger and/or better telescope using more modest priced eyepieces than you will with the most expensive eyepieces and a lesser telescope.

The one thing to remember when you choose an eyepiece is that no one eyepiece is perfect for all possible uses. An Ultra-wide angle eyepiece may be the best for wide field views of deep-sky objects, but it is not the best choice to see details of the planets. With each purchase of an eyepiece, think about what you are going to view with it. That should help you pick a good eyepiece for a specific purpose.

Considerations

What power do I get? The power of the telescope is calculated by dividing the focal length of the main mirror or lens by the focal length of the eyepiece used. For example, an 8" SCT at f/10 (2000mm focal length) will get 200X (200 power) with a 10mm focal length eyepiece, 80X with a 25mm eyepiece and 50X with a 40 mm eyepiece.

What power should I use? People often think that the highest powers possible is the best power to use. Actually the lower power (longer focal length) eyepieces are always easier to use than the high power (shorter focal length) eyepieces. The lowest power that will do the job is the one to use. Remember that power magnifies everything. That includes shakiness in the mount and motion in the air stream. High powers also give dimmer views because the same amount of light is spread out over the larger image.

What is the lowest power to use? For large objects like large star clusters and portions of the Milky Way, wide field, low power views will show the most as the dictum: The lower the power, the larger the field of view prevails. But there is a lower limit depending on the size of your eye's pupil in the dark night. If the size of the light leaving the telescope (called the exit pupil) is larger than your eye pupil, you lose light. For a telescope, the exit pupil is about equal to the diameter of the telescope divided by the power. Because your eye pupil at night averages about 7mm, the exit pupil of this size gives you the most light. This comes out to about 3.5X per inch of telescope.

What is the highest power to use? On any night, unsteadiness in the air might limit the maximum power you can use. Assuming the air is steady, the telescope is good, and the mount is steady, the highest power that is useful depends on contrast. For a high contrast object like a double star, you can use up to about 100X per inch of telescope under the best of conditions. For low contrast details like the planets, about 40X to 50X per inch is a maximum. On nights where the atmosphere is very unsteady, it may not be possible to use even 25X per inch.

The higher powers on low contrast objects lose contrast because the object gets too dim. You should try several powers on each object each night to see what works best at that time. On large objects like the North American Nebula, higher powers will lose the entire object because you cannot see the entire nebula against darker sky background. In the same way, details on the moon's seas are lost when the sun is high at the feature. This is because there is no contrast between their background and the brighter regions around the seas.

What is the field of view? There are two different fields of view defined. One of these is the apparent field of view (AFOV). This is the angle your eye appears to view when it looks through the eyepiece. Most Plössl type of eyepieces have an AFOV of about 52°. This is about the maximum for a four-element eyepiece. The longest focal length eyepieces have a smaller AFOV due to the fact that the 1-1/4" eyepiece mount restricts the field of view.

The 40mm has about 43° AFOV. Five-element eyepieces may have AFOV’s of 60°, 65° and 70°. These eyepieces make it easier to see more of the larger objects and still have a higher power. These are usually more expensive than “standard” eyepieces.

What is eye relief? The eye relief is the distance from the back of the eyepiece to where the image forms. For a standard four-element Plössl design, the eye relief is usually a little shorter than the focal length of the eyepiece. In general a long eye relief can be easier to use especially if you use glasses when you observe.

When you use a very long focal length eyepiece, such as a 40mm focal length, you might have to get adjusted to the long eye relief. The 40mm has about a 32mm eye relief, which is about 1-1/4". Your eye must be about 1-1/4" behind the top of the eyepiece to see the image.

For very short focal length eyepieces such as the 4mm Plössl, the eye relief can be too short to use glasses with the eyepieces. For any Plössl eyepiece, regardless of the brand or price, the eye relief is about equal to 3/4 of the focal length - 3mm for 4mm Plössl. It almost feels that your eye must be inside the eyepiece with these high power eyepieces. Try taking the rubber eyecup off in these cases. This is another problem with using higher powers than are necessary.

The wide field eyepieces also have great eye relief at the higher powers. This is another major advantage with their use.

Contrast in very important to bring out faint deep-sky features and fine planetary detail. Consider eyepieces that have edge-blackened optics and advanced multi-coated optics. These additional steps in the manufacturing process result in better contrast with your telescope. In any view through a telescope, especially where a bright star or planet is present, there is some scattering of light.

With standard optics some of this light may be reflected back into the eyepiece and this results in decreased contrast. With edge-blackened optics, this reflection is eliminated.

Accesories

A Barlow lens multiplies your eyepiece power. A 2X Barlow will give you twice the magnification than the eyepiece alone, a 3X, three times, etc. In addition the Barlow lens keeps the eye relief of the original eyepiece. It is MUCH more pleasant (at least for me) to use a 2X Barlow with a 9mm Plössl eyepiece than using a 4mm eyepiece and still get the magnification benefit.